Normal reaction doubt

  • #1

Homework Statement


My question is not a homework problem. I am having difficulty in understanding the concept oof contact forces.

Homework Equations


What I don't understand is that when two bodies A and B are in contact and left on a frictionless surface, they apply normal reactions on each other, then why doesn't that normal reaction force give some acceleration to the blocks and they move away from each other?

The Attempt at a Solution


The only viable solution I can come up with is that the normal reaction is zero, but they why it would be zero I don't know.
 

Answers and Replies

  • #2
kuruman
Science Advisor
Homework Helper
Insights Author
Gold Member
9,946
3,112

Homework Statement


My question is not a homework problem. I am having difficulty in understanding the concept oof contact forces.

Homework Equations


What I don't understand is that when two bodies A and B are in contact and left on a frictionless surface, they apply normal reactions on each other, then why doesn't that normal reaction force give some acceleration to the blocks and they move away from each other?

The Attempt at a Solution


The only viable solution I can come up with is that the normal reaction is zero, but they why it would be zero I don't know.
When you say they "are in contact and left on a frictionless surface", do you mean that they are moving at constant velocity or accelerating or what? Consider the picture shown below. Let's say that the hand exerts constant force F to the right. The two masses will accelerate to the right with acceleration ##a=F/(m_A+m_B)##.
Block A experiences two horizontal forces, one from the hand and one from Block B. Block B experiences only one horizontal force and that is the contact force exerted on it by block A. It is that contact force that accelerates block B. Now suppose that the hand stops pushing when the blocks reach speed v. Do you see why the contact force is zero after the hand stops pushing?

handon blocks.jpg
 

Attachments

  • #3
haruspex
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
35,509
6,421
the normal reaction is zero, but they why it would be zero
It helps to start with a definition of normal force: it is that force of minimum magnitude necessary to inhibit interpenetration of the objects. In the absence of any force tending to push one into the other there is therefore no normal force.
 
Last edited:
  • Like
Likes Vatsal Goyal

Related Threads on Normal reaction doubt

  • Last Post
Replies
6
Views
881
Replies
5
Views
1K
  • Last Post
Replies
2
Views
5K
Replies
1
Views
8K
Replies
6
Views
2K
Replies
5
Views
1K
Replies
5
Views
9K
Replies
6
Views
2K
Replies
1
Views
3K
Top